Substantial attention has been directed over many years to the design of disposable devices for producing a mist of medication and delivering it to the airways of a patient under treatment. One important application for such devices is a treatment for pneumocystis carinii pneumonia, a common infection of patients afflicted with acquired immune deficiency syndrome. The delivery of a fine mist of pentamidine isethionate deeply into the lungs of a patient is a recommended therapy for this form of pneumonia.
Devices presently in use for delivery of pentamidine in aerosol form have proven to be relatively inefficient. In a typical 300 mg. dosage treatment, only a minor fraction of the medication actually is deposited in the lungs. Substantial portions are vented to the expiratory filter or otherwise left as unusable residue in the devices. With expensive drugs such as pentamidine, this inefficiency adds substantially to the cost of therapy. The medication cost for a typical dosage of pentamidine to the patient is on the order of $300.
A most widely used device for pentamidine delivery is the Respirgard II marketed by Marquest Medical Products of Inglewood, Colo. The Respirgard II product is substantially similar to the disposable devices illustrated in U.S. Patent No. 4,823,784, particularly the one in FIG. 4. The breathing circuit defined by the Respirgard device and the patent is separated between two distinct legs, one for inhalation and one for exhalation. The aerosol medication from a nebulizer joins the inhalation conduit downstream from an ambient air intake. Check valves are provided in the inhalation conduit on both sides of the nebulizer. During inhalation, the patient draws in air both from the atmosphere and the nebulizer through the inhalation conduit. The exhalation conduit is provided with a check valve and filter to receive all air flow from patient exhalation. The check valve in the exhalation leg prevents any intake of air through that leg during inhalation.
One result from this breathing circuit arrangement is that, if the aerosol medication continues to be produced from the nebulizer during patient exhalation, substantial amounts of medication will be lost to the expiratory filter by being expelled with the exhalent. U.S. Patent No. 4,823,784, in FIG. 1, shows a T provided at the inlet of the nebulizer with a vent port so that the aerosol will only be produced when the patient or a clinician places a finger over the port. Such an arrangement complicates usage, and has not been widely employed.
Moreover, the breathing circuitry of the state of the art device does not inherently encourage the slow deep breathing which is desired for maximum effectiveness in this type of therapy. Patients tend to take rapid and relatively shallow breaths, and unless carefully coached and supervised, generally do not take deep, slow breaths, failing to optimize their treatment. In addition, the devices of the prior art have a tendency to waste substantial amounts of the medication simply by deposition of the medication on the walls of the nebulizer.